BibDocFile: fix for BibDocFile instantiation

* BibDocFile was recently refactored so that it no longer takes a
  doctype parameter on initialization.  However the function
  generic_path2bidocfile was still wrongly initializing BibDocFile
  with the this parameter.  This patch fixes the problem.
  (closes #1317)

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>